Prior antenna devices attached to vehicles are ordinarily arranged as antenna devices which can receive AM broadcasts and FM broadcasts. As these antenna devices, there is known a vehicle-mounted antenna device arranged as a helical antenna having an antenna rod portion around which a helical element is helically wound.
FIG. 29 is a perspective view showing an example of an arrangement of the antenna device. In the antenna device 100 shown in FIG. 29, a helical element 111 is wound around the outer periphery of a rod-shaped insulation support member 110 at pitches p. A metal element fitting 113 is fitted with the lower end of the support member 110. The lower portion of the element fitting 113 is arranged as an attachment portion 114 for fixing the antenna device 100 to an antenna case attached to a roof and the like of a vehicle, and a male screw, for example, is formed on the attachment portion 114. Further, although not shown, the portion from the extreme of the support member 110, around which the helical element 111 is wound, to the element fitting 113 is molded with resin.
FIG. 30 shows the frequency characteristics of a voltage standing wave ratio (VSWR) in the frequency band of FM broadcast when the antenna device 100, which is composed of the support member 110, around which the helical element 111 is wounded, having a diameter φ of 6.8 mm, with pitches p of 1.48 mm, and a length L of 154 mm, is attached to the antenna case. Note that the length L1 of the element fitting 113 is set to about 22.5 mm. Referring to the frequency characteristics shown in FIG. 30, it can be seen that the antenna device 100 approximately resonates with 83 MHz which is the center frequency of FM broadcast. Since the frequency band of FM broadcast in Japan is set from 76 MHz to 90 MHz, it can be found that the antenna device 100 shown in FIG. 29 is approximately operated in the frequency band of FM broadcast. Incidentally, it is recently desired to receive terrestrial digital television broadcast also by vehicles. The frequency band of the terrestrial digital televisions employs an UHF band from 470 MHz to 710 MHz. FIG. 31 shows the frequency characteristics of the UHF band when the antenna device 100 shown in FIG. 29 is attached to the antenna case. Referring to FIG. 31, it can be found that the antenna device 100 does not operate in the UHF band and cannot receive terrestrial digital television broadcast, although this is a matter of course because the antenna device 100 is arranged to receive FM broadcast.
FIG. 32 shows an arrangement example of a prior multi-frequency antenna which is operated in a plurality of frequency bands.
In the multi-frequency helical antenna 200 shown in FIG. 32, the length of a passive coil portion 216, in which the number of windings is increased, is adjusted so that operation frequency band thereof is set to a 800 MHz band in a mobile phone network, and the length of a second passive coil portion 219, in which the number of windings is slightly reduced, is adjusted so that the operation frequency band thereof is set to the vicinity of the 800 MHz band in the mobile phone network. With this arrangement, a sufficiently wide frequency band can be secured even in the low frequency band of the 800 MHz. Further, when the length of an exciter coil portion 217, in which the number of windings is reduced, is adjusted so that the operation frequency band thereof is set to a 1.5 GHz band in the mobile phone network, the multi-frequency helical antenna 200 can be operated in the 800 MHz band and the 1.5 GHz band in the mobile phone network. Note that the passive coil portion 216 and the second passive coil portion 219 are excited by the exciter coil portion 217.    [Patent Document 1] Japanese Publication Unexamined Patent Application No. 2000-295017    [Patent Document 2] Japanese Publication Unexamined Patent Application No. 2003-37426